This application is being concurrently filed with the following applications:
TI-17611, Ser. No. 08/038,391 "GRAY SCALE PRINTING USING SPATIAL LIGHT MODULATORS"; and PA1 TI-17335, Ser. No. 08/038,392 "COMBINED MODULATOR SCHEME FOR SPATIAL LIGHT MODULATORS".
1. Field of the Invention
This invention relates to printers, more particularly to printers using spatial light modulators (SLMs).
2. Background of the Invention
An array of individual elements typically comprise a spatial light modulator. Examples of these arrays are Liquid Crystal Devices (LCDs), and Digital Micromirror Devices (DMDs). Some SLMs are binary, that is, their cells are either ON or OFF, others may have a set number of levels. In printing systems that produce gray scales, this causes some problems.
Xerographic printing systems typically use the following process. The print engine receives the data from the source image, such as a computer terminal, or a scanning unit that has scanned an image for reproduction. The printing system converts it to light information, and impinges the light in the correct patterns on a photosensitive drum, causing latent charge patterns on the drum. The drum then rotates past a toner dispenser and attracts toner particles to the latent image on the drum's surface. The drum rotates with these toner particles and transfers them onto substrate, typically a piece of paper. The toner is then fused to the paper, usually with heat, and the paper exits the print engine.
Currently, most of such systems use scanned laser exposure systems for high quality printing. Spatial light modulators are preferred because they potentially offer more functionality for printing. Laser scanning systems print pixel-by-pixel by scanning the laser beam along raster lines. For example, if the user wants to print an 8.5"by 11" paper with 600 dots per inch (dpi) at 40 pages per minute, each page requires 11".times.600 dpi, or 6600 lines. At 40 pages per minute this equals 4400 lines per second, where each line has 8.5.times.600, or 5100 pixels. This requires 22.44.times.10.sup.6 pixels per second, allowing 44.6 nanoseconds per pixel. Lasers cannot modulate fast enough in an analog fashion to give gray-scale exposure during the short pixel time. Since SLMs have many light modulating elements that can operate simultaneously, they can be configured to print an entire line of print information at a time. The number of pixels printed per second remains the same, but the SLM approach provides a time that is 5100 times longer ( 5100.times.44.6 nanoseconds=227.5 microseconds) to print individual pixels with gray scale modulation.
However, SLMs have their own problems. Because of the nature of their ON/OFF or set number of levels operating mode, production of gray-scale, or shades of gray in between full ON (black) and OFF (no toner) images, becomes very difficult. Each element has the advantages of consistent spot size, shape, and location that makes good quality with sharp images. However, SLM arrays with easily manufacturable resolutions, e.g. 300 dpi, have difficulty rendering curved graphics and fine text features because of the fixes levels of exposure and pixel location limitations.
Therefore, a solution to the above problems offers an opportunity to make printing systems with better quality, lower price, and easier maintainability for more consistent performance.